Local Extinction

In 1997 the Jasper Ridge population of the bay checkerspot declined to extinction [1]. The discovery of the large population near Morgan Hill in the mid-1980's made Ehrlich, Murphy, and colleagues believe that they could learn more from watching the Jasper Ridge population decline to extinction than by intervening to prevent its extinction.

In a similar vein, populations of a different subspecies of Euphydryas editha underwent a drastic change in the central Sierra Nevada [4]. Clearcutting of the forest in part of Sequoia National Forest in 1967 created artificial meadows that were excellent habitat, and the butterflies switched to a different food plant that was very abundant there, Collinsia torreyi.

• Populations in clear-cut areas behaved as source populations.
  • They produced an excess of offspring every year, and some of that excess resulted in migrants to nearby native populations on rock outcrops.

  • Population sizes on outcrops were larger the closer those populations were to clearcut populations.

  • Individual breeding success was poor in outcrop populations.

• During a severe drought in 1992, plants of C. torreyi did not survive, but the host plants used by checkerspots in outcrop populations persisted.

• Clear-cut populations of checkerspot declined to extinction, while outcrop populations persisted.